Lash adjuster

ABSTRACT

The object is to prevent the adjusting screw from being pushed in while rotating when a load is applied to the adjusting screw that tends to push in the adjusting screw. 
     In a lash adjuster comprising a cylindrical housing  12  inserted in a receiving hole  11  formed in the top surface of a cylinder head  2  and having an internal thread  13  on the inner periphery thereof, an adjusting screw  15  having an external thread  14  on the outer periphery thereof and in threaded engagement with the internal thread  13  on the housing  12 , and a return spring  17  biasing the adjusting screw  15  in a direction to protrude upwardly from the housing  12 , the adjusting screw  15  having a protruding end  19  protruding from the housing  12  and supporting an arm  7 , the surface of the protruding end  19  of the adjusting screw  15  is made of a material that does not react with oil additives of oil containing organic molybdenum.

TECHNICAL FIELD

This invention relates to a lash adjuster mounted in a valve gear of an engine.

BACKGROUND ART

One known valve gear for moving the valve at an engine intake port or exhaust port includes an arm pivotable about one end thereof and adapted to be pushed down at its central portion by a cam, thereby pushing down the valve stem at its other end.

With this valve gear, gaps between the component parts of the valve gear tend to change while the engine is running due to differences in thermal expansion between the component parts of the valve gear, which may lead to noise or pressure leakage.

In order to prevent such noise and pressure leakage, a lash adjuster is frequently mounted in the valve gear to absorb changes in size of gaps between component parts of the valve gear.

Typically, such lash adjusters comprise a cylindrical housing inserted in a receiving hole formed in the top surface of a cylinder head and having an internal thread on the inner periphery thereof, an adjusting screw having an external thread on the outer periphery thereof and in threaded engagement with the internal thread on the housing, and a return spring biasing the adjusting screw in a direction to protrude upwardly from the housing (Patent documents 1 and 2).

In Patent document 1, the return spring is a compression coil spring that applies an axial force to the adjusting screw that tends to push the adjusting screw out of the housing.

With these lash adjusters, the protruding end of the adjusting screw protruding from the housing is hemispherical and is received in a recess formed in the arm of the valve gear, thereby supporting the arm so as to be pivotable about the protruding end of the adjusting screw. When the cam rotates and the arm pivots, a load is applied to the adjusting screw that tends to push the adjusting screw into the housing. But due to the frictional resistance between the external thread on the adjusting screw and the internal thread on the housing, the adjusting screw is prevented from rotating, so that the adjusting screw is fixed in position in the axial direction.

Further, the radially outer portion of the protruding end of the adjusting screw and the inner surface of the recess of the arm are spherical surfaces having the same radius and having their centers located on the axis of the adjusting screw. Thus, due to the frictional resistance between these spherical surfaces too, the rotation of the adjusting screw is prevented.

With this type of lash adjusters, when the relative position between the arm and the cylinder head changes due e.g. to thermal expansion of the valve gear, according to the amount of the change in relative position, the adjusting screw moves axially while rotating in the housing, thus absorbing the changes in gaps between component parts of the valve gear.

Patent document 1: JP Patent Publication 2005-273510A Patent document 2: JP Utility Model Publication 64-34407

DISCLOSURE OF THE INVENTION Object of the Invention

In many cases, as engine oil for lubricating sliding surfaces of the engine, engine oil containing organic molybdenums (hereinafter referred to as “FM oil”) is used. If FM oil is used, lubricating film containing molybdenum disulfide (MoS₂) forms on sliding surfaces of ferrous metal. Such lubricating film serves to reduce the friction coefficient of the sliding surfaces. Organic molybdenums include molybdenum dithiocarbamate (MoDTC) and molybdenum dithiophosphate (MoDTP).

The adjusting screw of the above-mentioned lash adjusters is made of iron to ensure rigidity required for the screw. Thus, if FM oil is used as engine oil, lubricating film forms on the adjusting screw. The lubricating film thus formed reduces the frictional resistance between the protruding end of the adjusting screw protruding from the housing and the recess of the arm, as well as the frictional resistance between the external thread on the adjusting screw and the internal thread on the housing. As a result, when the cam rotates and a load is applied to the adjusting screw that tends to push the adjusting screw into the housing, the adjuster screw tends to rotate and pushed into the housing. This may reduce the valve lift amount or cause the valve to be impulsively seated on the valve seat, thus producing noise.

Similarly, if the arm of the lash adjuster is made of iron too, lubricating film formed on the surface of the arm tends to reduce the frictional resistance between the protruding end of the adjusting screw protruding from the housing and the recess of the arm, thus allowing the adjusting screw to be more easily pushed in while rotating when the cam rotates and a load is applied to the adjusting screw that tends to push the adjusting screw into the housing.

If wear progresses of the external thread of the adjusting screw and the internal thread of the housing, the adjusting screw may be pushed into the housing while rotating while the engine is rotating, thus reducing the valve lift or causing the valve to be impulsively seated on the valve seat, thus producing noise.

An object of the present invention is to prevent rotation of the adjusting screw when a load is applied to the adjusting screw that tends to push in the adjusting screw.

MEANS TO ACHIEVE THE OBJECT

In order to achieve this object, the surface of the protruding end of the adjusting screw is made of a material that does not react with oil additives of oil containing organic molybdenum.

In order to make the surface of the protruding end of the adjusting screw non-reactive with oil additives of oil containing organic molybdenum, the adjusting screw may be made of a non-ferrous metal that does not react with oil additives of oil containing organic molybdenum. Such a non-ferrous metal may be titanium or aluminum.

If the adjusting screw is made of iron, in order to make the surface of the protruding end of the adjusting screw non-reactive with oil additives of oil containing organic molybdenum, the protruding end of the adjusting screw may be coated with a nonreactive film selected from ceramic film, carbon film, oxide film, diamond-like carbon film, titanium nitride film and chromium nitride film.

If the adjusting screw is made of iron, in order to make the surface of the protruding end of the adjusting screw non-reactive with oil additives of oil containing organic molybdenum, a nitride compound layer may also be formed on the protruding end of the adjusting screw.

If the adjusting screw is made of iron, in order to make the surface of the protruding end of the adjusting screw non-reactive with oil additives of oil containing organic molybdenum, nickel-phosphorus plating may also be provided on the protruding end of the adjusting screw. In order to improve the wear resistance of the protruding end of the adjusting screw, such nickel-phosphorus plating may be dispersion plating comprising a plating layer in which hard particulates (of e.g. SiC or Si₃N₄) are dispersed. Also, such nickel-phosphorus plating may be dispersion plating comprising a plating layer in which PTFE is dispersed.

If the arm of the valve gear is made of iron, in order to achieve the object of the invention, the inner surface of the recess of the arm may be made of a material that does not react with oil additives of oil containing organic molybdenum.

In this case too, in order to make the inner surface of the recess of the arm non-reactive with oil additives of oil containing organic molybdenum, the recess of the arm may be coated with a nonreactive film selected from ceramic film, carbon film, oxide film, diamond-like carbon film, titanium nitride film and chromium nitride film.

In order to make the inner surface of the recess of the arm non-reactive with oil additives of oil containing organic molybdenum, a nitride compound layer may also be formed on the inner surface of the recess of the arm.

In order to make the inner surface of the recess of the arm non-reactive with oil additives of oil containing organic molybdenum, nickel-phosphorus plating may also be provided on the recess of the arm. In order to improve the wear resistance of the recess of the arm, such nickel-phosphorus plating may be dispersion plating comprising a plating layer in which hard particulates are dispersed. Also, such nickel-phosphorus plating may be dispersion plating comprising a plating layer in which PTFE is dispersed.

The object of the present invention can also be achieved with the arrangement in which as taken along a plane containing the axis of the adjusting screw, the recess of the arm has a section whose radius of curvature is larger than the radius of curvature of the protruding end of the adjusting screw, whereby the protruding end possesses the wedging effect.

Specifically, this lash adjuster may be structured as follows:

1) The recess of the arm has an arcuate section as taken along a plane containing the axis of the adjusting screw, and has centers of arc that are offset from the axis of the adjusting screw such that a radius of arc of the recess intersects the axis of the adjusting screw, and the protruding end of the adjusting screw has an arcuate section having a center located on the axis of the adjusting screw, as taken along a plane containing the axis of the adjusting screw. 2) The recess of the arm has an arcuate section having a center located on the axis of the adjusting screw, as taken along a plane containing the axis of the adjusting screw, and the protruding end of the adjusting screw has an arcuate section as taken along a plane containing the axis of the adjusting screw, and has centers of arc that are offset from the axis of the adjusting screw such that a radius of arc of the protruding end is separate from the axis of the adjusting screw. 3) The recess of the arm has an arcuate section as taken along a plane containing the axis of the adjusting screw, and has centers of arc that are offset from the axis of the adjusting screw such that a radius of arc of the recess intersects the axis of the adjusting screw, and the protruding end of the adjusting screw has an arcuate section as taken along a plane containing the axis of the adjusting screw, and has centers of arc that are offset from the axis of the adjusting screw such that a radius of arc of the protruding end is separate from the axis of the adjusting screw.

With such a lash adjuster, a cutout may be formed in the inner surface of the recess of the arm at its central portion.

In order to achieve this object, the present invention also provides a lash adjuster in which a recessed surface or a flat surface is formed on the protruding end of the adjusting screw at its central portion, the recessed surface or the flat surface having a diameter which is half or more than half the diameter of the protruding end of the adjusting screw.

If the return spring is a compression coil spring adapted to apply an axial force to the adjusting screw that tends to push the adjusting screw out of the housing, the external thread and the internal thread may be serration-shaped threads.

If the return spring is a torsion spring adapted to apply torque to the adjusting screw that tends to push the adjusting screw out of the housing, the external thread and the internal thread may be serration-shaped threads, triangular threads or trapezoidal threads. The torsion spring as the return spring may be a torsion coil spring, a spiral spring or a volute spring.

The adjusting screw may comprise a pivot member axially slidably inserted in the housing, an externally threaded member supporting an end of the pivot member inserted in the housing and having the external thread on its outer periphery, and an elastic member disposed between the pivot member and the externally threaded member.

ADVANTAGES OF THE INVENTION

With the lash adjuster according to the present invention of which the surface of the protruding end of the adjusting screw is made of a material that does not react with oil additives of oil containing organic molybdenum, lubricating film containing molybdenum disulfide is less likely to form on the surface of the protruding end of the adjusting screw protruding from the housing, so that the friction between the adjusting screw and the arm is less likely to decrease. This prevents the adjusting screw from being pushed into the housing while rotating when the cam rotates and a load is applied to the adjusting screw that tends to push in the adjusting screw.

With the lash adjuster according to the present invention of which the inner surface of the recess of the arm is made of a material that does not react with oil additives of oil containing organic molybdenum, too, lubricating film containing molybdenum disulfide is less likely to form on the inner surface of the recess of the arm, so that the friction between the adjusting screw and the arm is less likely to decrease. This prevents the adjusting screw from being pushed into the housing while rotating when the cam rotates and a load is applied to the adjusting screw that tends to push in the adjusting screw.

With the lash adjuster according to the present invention of which the recess of the arm has a radius of curvature larger than the radius of curvature of the protruding end of the adjusting screw, whereby the protruding end possesses the wedging effect, when the surface of the recess is pressed against the protruding end of the adjusting screw, the protruding end is wedged against the surface of the recess. Thus, while the engine is running, large frictional resistance is produced between the radially outer portion of the protruding end and the inner surface of the recess. This frictional resistance effectively prevents rotation of the adjusting screw.

With the lash adjuster according to the present invention in which a recessed surface or a flat surface is formed on the protruding end of the adjusting screw at its central portion, with the recessed surface or the flat surface having a diameter which is half or more than half the diameter of the protruding end of the adjusting screw, the protruding end of the adjusting screw contacts the inner surface of the recess of the arm at its portion radially outwardly of the recessed surface or the flat surface. Thus, larger frictional resistance is produced between the protruding end and the recess, so that it is possible to effectively prevent rotation of the adjusting screw 15 with the increased frictional resistance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front view of a valve gear in which is mounted a lash adjuster according to a first embodiment of the present invention.

FIG. 2 is an enlarged sectional view of a protruding end of the adjusting screw of FIG. 1.

FIG. 3 is an enlarged sectional view of a modification of the adjusting screw shown in FIG. 2.

FIG. 4 is an enlarged sectional view of the arm shown in FIG. 3.

FIG. 5 is an enlarged sectional view of a lash adjuster according to a second embodiment of the present invention.

FIG. 6 is an enlarged sectional view of a modification of the lash adjuster shown in FIG. 5.

FIG. 7 is an enlarged sectional view of a lash adjuster according to a third embodiment of the present invention.

FIG. 8 is an enlarged sectional view of a lash adjuster according to a fourth embodiment of the present invention.

FIG. 9 is a sectional view taken along line IX-IX of FIG. 8.

FIG. 10 is an enlarged sectional view of a lash adjuster according to a fifth embodiment of the present invention.

FIG. 11 is an enlarged sectional view of a lash adjuster according to a sixth embodiment of the present invention, showing the protruding end of its adjusting screw.

FIG. 12 is an enlarged sectional view of a lash adjuster according to a seventh embodiment of the present invention, showing the protruding end of its adjusting screw.

FIG. 13 is an enlarged sectional view of a modification of the lash adjuster shown in FIG. 10, showing the protruding end of its adjusting screw.

FIG. 14 is an enlarged sectional view of a lash adjuster according to an eighth embodiment of the present invention, showing the protruding end of its adjusting screw.

DESCRIPTION OF NUMERALS

-   1. Lash adjuster -   2. Cylinder head -   7. Arm -   11. Receiving hole -   12. Housing -   13. Internal thread -   14. External thread -   15. Adjusting screw -   15A. Pivot member -   15B. Externally threaded member -   15C. Spring washer -   17. Return spring -   19, 61, 71, 81, 91. Protruding end -   20, 62, 72, 82, 92. Recess -   22, 23. Nonreactive film -   63. Cutout -   93. Recessed surface -   A1. Radius of the protruding end -   A2. Radius of the recess -   B1. Radius of arc of the protruding end -   C1. Radius of arc of the protruding end -   C2. Radius of arc of the recess -   D1. Diameter of the recessed surface -   D2. Diameter of the protruding end -   L. Axis of the adjusting screw

BEST MODE FOR EMBODYING THE INVENTION

FIG. 1 shows a valve gear including a lash adjuster 1 according to the first embodiment of the present invention. The valve gear includes a valve 4 provided at an intake port 3 of a cylinder head 2 of an engine, a valve stem 5 connected to the valve 4, and an arm 7 pivotable along with the rotation of a cam 6.

The valve stem 5 extends upwardly from the valve 4, slidably extending through the cylinder head 2. An annular spring retainer 8 is fixed to the outer periphery of the valve stem 5 at its top end portion. A valve spring 9 is mounted between the bottom surface of the spring retainer 8 and the top surface of cylinder head 2. The valve spring 9 biases the valve stem 5 upwardly through the spring retainer 8, thereby seating the valve 4 on a valve seat 10.

As shown in FIG. 2, the lash adjuster 1 comprises a cylindrical housing 12 inserted in a receiving hole 11 formed in the top surface of the cylinder head 2, an adjusting screw 15 having an external thread 14 formed on the outer periphery thereof at its bottom portion and kept in threaded engagement with an internal thread 13 formed on the inner periphery of the housing 12, a bottom member 16 fixed to the bottom end of the housing 12, and a return spring 17 mounted between the adjusting screw 15 and the bottom member 16.

The external thread 14 and the internal thread 13 have an asymmetrical section taken along the axis, and have pressure flanks for receiving pressure when a load is applied that tends to push the adjusting screw 15 into the housing 12, the pressure flanks having a larger flank angle than their respective clearance flanks.

The return spring 17 is a compression coil spring having its bottom end supported by the bottom member 16 and having its top end pressed against the adjusting screw 15 through a spring seat 18, thereby applying an axial force to the adjusting screw 15 that tends to push the adjusting screw out of the housing 12. The axial force thus tends to bias the adjusting screw 15 in the direction to protrude from the housing 12.

As shown in FIG. 1, the arm 7 has a recess 20 in its bottom surface at one end thereof in which the end 19 of the adjusting screw 15 protruding from the housing 12 engages, and thus is supported so as to be pivotable about the protruding end 19 of the adjusting screw 15. The bottom surface of the arm 7 at its other end is kept in contact with the top end of the valve stem 5. A roller 21 is mounted on the arm 7 at its central portion so as to be in contact with the overhead cam 6.

As shown in FIG. 2, the protruding end 19 of the adjusting screw 15 is hemispherical in shape. The adjusting screw 15 is made of a non-ferrous metal that does not react with oil additives of FM oil (such as titanium or aluminum). Thus, even if FM oil is used as engine oil, no lubricating film containing molybdenum disulfide (MoS₂) forms on the surface of the protruding end 19 of the adjusting screw 15. The arm 7 is made of an aluminum alloy.

Now the operation of the lash adjuster 1 is described.

When the cam 6 is rotated by the engine and the arm 7 is pushed down by the lobe 6 a of the cam 6, the valve 4 separates from the valve seat 10, thus opening the intake port 3. At this time, while a load is applied to the adjusting screw 15 that tends to push in the screw 15, the rotation of the adjusting screw 15 is prevented by the frictional resistance between the external thread 14 of the adjusting screw 15 and the internal thread 13 of the housing 12, as well as by the frictional resistance between the protruding end 19 of the adjusting screw 15 and the recess 20 of the arm 7. Thus, the axial position of the adjusting screw 15 is fixed.

When the cam 6 further rotates and the cam lobe 6 a separates from the roller 21, the valve stem 5 rises under the biasing force of the valve spring 9, seating the valve 4 on the valve seat 10, thereby closing the intake port 3.

Strictly speaking, when the arm 7 is pushed down by the lobe 6 a of the cam 6, slight slip occurs between the pressure flank of the external thread 14 and the pressure flank of the internal thread 13, which causes the adjusting screw 15 to be pushed in. But when the cam lobe 6 a moves past the roller 21 and the load that tends to push in the adjusting screw 15 is removed, the adjusting screw 15 protrudes under the biasing force of the return spring 17 and moves back to its original position.

If the distance between the cam 6 and the arm 7 increases due to differences in thermal expansion coefficient among the component parts of the valve gear, including the cylinder head 2, valve stem 5 and arm 7, the adjusting screw 15 protrudes by a larger amount when the cam 6 further rotates after the arm 7 has been pushed down by the lobe 6 a of the cam 6 and the load that tends to push in the adjusting screw is removed, than the amount by which the adjusting screw 15 is pushed into the housing by the lobe 6 a of the cam 6. Thus, every time the cam 6 rotates once, the adjusting screw 15 gradually protrudes from the housing, thereby preventing the formation of a gap between the base circle 6 b and the roller 21.

Conversely, if the contact surfaces of the valve 4 and the valve seat 10 become worn, since the biasing force of the valve spring 9 acts on the adjusting screw 15 even while the base circle 6 b of the cam 6 faces the roller 21. Thus, the adjusting screw 15 protrudes by a smaller amount when the cam 6 further rotates after the arm 7 has been pushed down by the lobe 6 a of the cam 6 and the load that tends to push in the adjusting screw is removed, than the amount by which the adjusting screw 15 is pushed into the housing by the lobe 6 a of the cam 6. As a result, every time the cam 6 rotates once, the adjusting screw 15 is gradually pushed into the housing, and thus the valve stem 5 gradually rises. This prevents formation a gap between the contact surfaces of the valve 4 and the valve seat 10.

With this lash adjuster 1, even if FM oil is used as engine oil, a lubricating film containing molybdenum disulfide is less likely to form on the surface of the end 19 of the adjusting screw 15 protruding from the housing 12, which makes the friction between the adjusting screw 15 and the arm 17 less likely to decrease. This in turn prevents the adjusting screw 15 from being pushed into the housing while rotating when the cam 6 rotates and a load is applied to the adjusting screw 15 that tends to push in the adjusting screw.

In this embodiment, in order to prevent the surface of the protruding end 19 of the adjusting screw 15 from reacting with oil additives of FM oil, the adjusting screw 15 is made of a material that does not react with oil additives of FM oil. But instead, as shown in FIG. 5, the adjusting screw 15 may be made or iron and the protruding end 19 of the adjusting screw 15 may be coated with a nonreactive film 22 selected from ceramic film, carbon film, oxide film, diamond-like carbon film, titanium nitride film and chromium nitride film.

In this case, the only the protruding end 19 of the adjusting screw 15 may coated with the nonreactive film 22. But preferably, the entire surface of the adjusting screw 15, including the external thread 14, is coated with the nonreactive film 22. With this arrangement, it is possible to prevent the formation of lubricating film containing molybdenum disulfide on the surface of the external thread 14, thereby ensuring frictional resistance between the external thread 14 and the internal thread 13.

Also, in order to prevent the surface of the protruding end 19 of the adjusting screw 15 from reacting with oil additives of FM oil, a nitride compound layer may be formed on the protruding end 19 of the adjusting screw 15 by subjecting the adjusting screw 15 to nitriding treatment.

Further, in order to prevent the surface of the protruding end 19 of the adjusting screw 15 from reacting with oil additives of FM oil, nickel-phosphorus plating may be provided on the protruding end 19 of the adjusting screw 15. Such nickel-phosphorus plating is preferably dispersion plating comprising a plating layer in which hard particulates (of e.g. SiC or Si₃N₄) are dispersed. Such plating can improve wear resistance of the protruding end 19 of the adjusting screw 15. The nickel-phosphorus plating may also be dispersion plating comprising a plating layer in which PTFE is dispersed.

In the above embodiment, the arm 7 is made of an aluminum alloy. But if the arm 7 is made of iron, as shown in FIG. 4, the recess 20 of the arm 7 is preferably coated with a nonreactive film 23 selected from ceramic film, carbon film, oxide film, diamond-like carbon film, titanium nitride film and chromium nitride film to prevent the inner surface of the recess 20 from reacting with oil additives of FM oil. With this arrangement too, even if FM oil is used as engine oil, a lubricating film containing molybdenum disulfide is less likely to form on the inner surface of the recess 20, so that the frictional resistance between the adjusting screw 15 and the arm 7 is less likely to decrease. This in turn prevents the adjusting screw 15 from being pushed into the housing while rotating when the cam 6 rotates and a load is applied to the adjusting screw 15 that tends to push in the adjusting screw.

Also, in order to prevent the inner surface of the recess 20 of the arm 7 from reacting with oil additives of FM oil, a nitride compound layer may be formed on the inner surface of the recess 20 of the arm 7 by subjecting the arm 7 to nitriding treatment.

Further, in order to prevent the inner surface of the recess 20 from reacting with oil additives of FM oil, nickel-phosphorus plating may be provided on the surface of the recess 20. Such nickel-phosphorus plating is preferably dispersion plating comprising a plating layer in which hard particulates (of e.g. SiC or Si₃N₄) are dispersed. Such plating can improve wear resistance of the recess 20. The nickel-phosphorus plating may also be dispersion plating comprising a plating layer in which PTFE is dispersed.

FIG. 5 shows the lash adjuster according to the second embodiment of the present invention. Elements identical to or corresponding to those of the first embodiment are denoted by identical numerals and their description is omitted.

The adjusting screw 15 comprises a pivot member 15A axially slidably inserted in the housing 12, an externally threaded member 15B supporting the end of the pivot member 15A inserted in the housing 12 and having an external thread 14 on its outer periphery, and a spring washer 15C disposed between the pivot member 15A and the externally threaded member 15B. The spring washer 15C may be a conical spring washer or a wave washer.

The pivot member 15A is made of a non-ferrous metal that does not react with oil additives of FM oil (such as titanium or aluminum). The externally threaded member 15B is also made of a non-ferrous metal that does not react with oil additives of FM oil.

The return spring 17 is a torsion coil spring having its bottom end engaged in an engaging hole 31 formed in the bottom member 16 and its top end engaged in an engaging hole 32 formed in the externally threaded member 15B. By being twisted, the return spring 17 applies torque to the externally threaded member 15B that tends to push the pivot member 15A out of the housing 12.

With this lash adjuster, as with the first embodiment, even if FM oil is used as engine oil, a lubricating film containing molybdenum disulfide is less likely to form on the surface of the end 19 of the pivot member 15A protruding from the housing 12, which makes the friction between the adjusting screw 15 and the arm 17 less likely to decrease. This in turn prevents the pivot member 15A and the externally threaded member 15B from being pushed into the housing while rotating when the cam 6 rotates and a load is applied to the pivot member 15A that tends to push in the pivot member.

Also, with this lash adjuster, since the externally threaded member 15B is made of a non-ferrous metal that does not react with oil additives of FM oil, a lubricating film containing molybdenum disulfide is less likely to form on the surface of the external thread 14. This ensures sufficient friction between the pivot member 15A and the arm 7, which in turn prevents the pivot member 15A and the externally threaded member 15B from being pushed into the housing while turning when the cam 6 rotates and a pushing force acts on the pivot member 15A.

With this lash adjuster, when the engine stops in heated condition and then the engine cools down, thus causing differences in shrinkage between the component parts of the valve gear, the spring washer 15C between the externally threaded member 15B and the pivot member 15A is compressed, thereby absorbing the differences in shrinkage. Thus, when the engine is restarted, no gap remains between the valve 4 and the valve seat 10 due to the differences in shrinkage between the component parts of the valve gear, thus preventing leakage of pressure.

In this embodiment, the spring washer 15C is used as an elastic member disposed between the pivot member 15A and the externally threaded member 15B. But instead of the spring washer 15C, a different elastic member (such as a compression coil spring) may be used.

If a torsion coil spring is used as the return spring 17, the return spring 17 may be a cylindrically wound one as shown in FIG. 5 or a conically wound one as shown in FIG. 6.

FIG. 7 shows the lash adjuster according to the third embodiment of the present invention, which differs from the second embodiment in that the return spring 17 of the second embodiment is replaced by a spiral spring. Here, elements corresponding to those of the second embodiment are denoted by identical numerals and their description is omitted.

The external thread 14 on the outer periphery of the externally threaded member 15B and the internal thread 13 on the inner periphery of the housing 12 are both vertically symmetrical triangular threads.

The return spring 17 is a spiral spring formed by helically winding a thin sheet material. The return spring 17 has its radially outer end rotationally fixed to the bottom member 16 and its radially inner end inserted in a slit formed in a protrusion 41 at the end of the externally threaded member 15B inserted in the housing 12. Thus, when the return spring 17 is twisted, the return spring 17 applies torque to the externally threaded member 15B that tends to push the pivot member 15A out of the housing 12.

With this lash adjuster, even if FM oil is used as engine oil, lubricating film containing molybdenum disulfide is less likely to form on the surface of the end 19 of the pivot member 15A protruding from the housing 12. This ensures sufficient friction between the pivot member 15A and the arm 7.

FIGS. 8 and 9 show a lash adjuster according to the fourth embodiment of the present invention. This embodiment differs from the second embodiment in that the return spring 17 of the second embodiment is replaced by a volute spring. Here, elements corresponding to those of the second embodiment are denoted by identical numerals and their description is omitted.

The external thread 14 on the outer periphery of the externally threaded member 15B and the internal thread 13 formed on the inner periphery of the housing 12 are both vertically symmetrical trapezoidal threads.

The return spring 17 is a volute spring formed by helically winding a thin sheet material. The return spring 17 has its radially outer end rotationally fixed to the bottom member 16 and its radially inner end inserted in a slit formed in a protrusion 51 at the end of the externally threaded member 15B inserted in the housing 12. Thus, when the return spring 17 is twisted, the return spring 17 applies torque to the externally threaded member 15B that tends to push the pivot member 15A out of the housing 12.

With this lash adjuster, even if FM oil is used as engine oil, lubricating film containing molybdenum disulfide is less likely to form on the surface of the end 19 of the pivot member 15A protruding from the housing 12. This ensures sufficient friction between the pivot member 15A and the arm 7.

In the second to fourth embodiments, the pivot member 15A is made of a material that does not react with oil additives of FM oil. But instead, the pivot member 15A may be made or iron and the protruding end 19 of the pivot member 15A may be coated with a nonreactive film 22 selected from ceramic film, carbon film, oxide film, diamond-like carbon film, titanium nitride film and chromium nitride film. With this arrangement, the material cost of the pivot member 15A is lower than when the pivot member 15A is made of a material that does not react with oil additives of FM oil.

Similarly, if the pivot member 15A is made of iron, a nitride compound layer may be formed on the surface of the protruding end 19 of the pivot member 15A or nickel-phosphorus plating may be provided on the protruding end 19 of the pivot member 15A. As nickel-phosphorus plating, dispersion plating may be used which comprises a plating layer in which hard particulates are dispersed to improve wear resistance of the protruding end 19 of the pivot member 15A.

The externally threaded member 15B may be also made of iron, and the surface of the externally threaded member 15B may be coated with a nonreactive film selected from ceramic film, carbon film, oxide film, diamond-like carbon film, titanium nitride film and chromium nitride film. With this arrangement, the material cost of the externally threaded member 15B is also lower than when the externally threaded member 15B is made of a material that does not react with oil additives of FM oil.

FIG. 10 shows a lash adjuster according to the fifth embodiment of the present invention. Elements corresponding to those of the first embodiment are denoted by identical numerals and their description is omitted.

The end 61 of the adjusting screw 15 protruding from the housing 12 is hemispherical in shape and is in contact with the inner surface of a recess 62 of the arm 7 at its radially outer portion. As taken along a plane containing the axis L of the adjusting screw 15, the protruding end 61 has an arcuate section of which the center is located on the axis L.

The recess 62 also has an arcuate sectional shape as taken along a plane containing the axis L of the adjusting screw 15. This arcuate recess 62 has centers of arc offset from the axis L so that its radius of arc A2 intersects the axis L. The radius of arc A2 of the recess 62 is larger than the radius of arc A1 of the protruding end 61. Thus, at the contact portion between the protruding end 61 and the recess 62, the radius of curvature of the recess 62 (=radius of arc A2) is larger than the radius of curvature of the protruding end 61 (=radius of arc A1). Due to this difference in radius of curvature, the wedging effect is produced at the protruding end 61.

With this lash adjuster, when the surface of the recess 62 is pressed against the protruding end 61 of the adjusting screw 15, the protruding end 61 is wedged against the surface of the recess 62 and tightened by the inner surface of the recess 62. Thus, while the engine is running, large frictional resistance is produced between the radially outer portion of the protruding end 61 and the inner surface of the recess 62. This frictional resistance effectively prevents rotation of the adjusting screw 15.

With this lash adjuster, since large frictional resistance is produced at the protruding end 61 of the adjusting screw 15, it is possible to reduce the frictional resistance between the external thread 14 and the internal thread 13. Thus, even if FM oil is used as engine oil or if wear of the external thread 14 and the internal thread 13 progresses, it is possible to prevent the adjusting screw 15 from being pushed into the housing while rotating while the engine is running.

Now the lash adjuster according to the sixth embodiment is described. Elements corresponding to those of the first embodiment are denoted by identical numerals and their description is omitted.

As shown in FIG. 11, the protruding end 71 of the adjusting screw 15 is hemispherical in shape and is in contact with the inner surface of a recess 72 of the arm 7 at its radially outer portion. As taken along a plane containing the axis L of the adjusting screw 15, the protruding end 71 has an arcuate section and has centers of arc offset from the axis L such that its radius of arc B1 is separate from the axis L.

As taken along a plane containing the axis L of the adjusting screw 15, the recess 72 has an arcuate section having a center located on the axis L. an arcuate section and has centers of arc offset from the axis L such that its radius of arc C1 is separate from the axis L. The radius of arc B2 of the recess 72 is larger than the radius of arc B1 of the protruding end 71. Thus, at the contact portion between the protruding end 71 and the recess 72, the radius of curvature of the recess 72 (=radius of arc B2) is larger than the radius of curvature of the protruding end 71 (=radius of arc B1). Due to this difference in radius of curvature, the wedging effect is produced at the protruding end 71.

With this lash adjuster, as in the fifth embodiment, when the surface of the recess 72 is pressed against the protruding end 71 of the adjusting screw 15, the protruding end 71 is wedged against the surface of the recess 72 and tightened by the inner surface of the recess 72. Thus, while the engine is running, large frictional resistance is produced between the radially outer portion of the protruding end 71 and the inner surface of the recess 72. This frictional resistance effectively prevents rotation of the adjusting screw 15.

The lash adjuster of the seventh embodiment according to the present invention is now described. Elements corresponding to those of the first embodiment are denoted by identical numerals and their description is omitted.

As shown in FIG. 12, the protruding end 81 of the adjusting screw 15 is hemispherical in shape and is in contact with the inner surface of a recess 82 of the arm 7 at its radially outer portion. As taken along a plane containing the axis L of the adjusting screw 15, the protruding end 81 has an arcuate section and has centers of arc offset from the axis L such that its radius of arc C1 is separate from the axis L.

As taken along a plane containing the axis L of the adjusting screw 15, the recess 82 also has an arcuate section and has centers of arc offset from the axis L such that its radius of arc C2 intersects the axis L. The radius of arc C2 of the recess 82 is larger than the radius of arc C1 of the protruding end 81. Thus, at the contact portion between the protruding end 81 and the recess 82, the radius of curvature of the recess 82 (=radius of arc C2) is larger than the radius of curvature of the protruding end 81 (=radius of arc C1). Due to this difference in radius of curvature, the wedging effect is produced at the protruding end 81.

With this lash adjuster, as in the fifth embodiment, when the surface of the recess 82 is pressed against the protruding end 81 of the adjusting screw 15, the protruding end 81 is wedged against the surface of the recess 82 and tightened by the inner surface of the recess 82. Thus, while the engine is running, large frictional resistance is produced between the radially outer portion of the protruding end 81 and the inner surface of the recess 82. This frictional resistance effectively prevents rotation of the adjusting screw 15.

With the lash adjuster of the fifth embodiment, as shown in FIG. 13, a cutout 63 may be formed in the inner surface of the recess 62 of the arm 7 at its central portion. The cutout 63 makes it easier for the recess to spread by elastic deformation. Thus, when the recess 62 is pressed against the protruding end 61, the protruding end 61 can be more easily wedged against the recess 62. A similar cutout may also be formed in the lash adjuster of the sixth or seventh embodiment.

The lash adjuster of the eighth embodiment according to the present invention is now described. Elements corresponding to those of the first embodiment are denoted by identical numerals and their description is omitted.

As shown in FIG. 14, the protruding end 91 of the adjusting screw 15 and a recess 92 formed in the arm 7 are both hemispherical in shape and have radii equal to each other. A recessed surface 93 is formed at the central portion of the protruding end 91 of the adjusting screw 15. The recessed surface 93 has a diameter D1 which is half or more than half the diameter D2 of the protruding end 91.

With this lash adjuster, the protruding end 91 of the adjusting screw 15 contacts the inner surface of the recess 92 of the arm 7 at its portion radially outwardly of the recessed surface 93. Thus, larger frictional resistance is produced between the protruding end 91 and the recess 92, so that it is possible to effectively prevent rotation of the adjusting screw 15 with the increased frictional resistance.

In this embodiment, the recessed surface 93 is formed at the central portion of the protruding end 91 of the adjusting screw 15. But the recessed surface 93 may be replaced by a flat surface having a diameter which is half or more than half the diameter D2 of the protruding end 91.

The lash adjuster of any of the above embodiments has the bottom member 16 fixed to the bottom end of the housing 12. But instead, the housing 12 may have an integral bottom at its bottom end.

The return spring 17 of any of the fifth to eighth embodiments may be replaced by any of the torsion springs of the second to fourth embodiments, which apply torque to the adjusting screw 15. The adjusting screw 15 of any of the fifth to eighth embodiments may be replaced by any of the adjusting screws of the second to fourth embodiments, which comprise the pivot member 15A, externally threaded member 15B and spring washer 15C. 

1. A lash adjuster comprising a cylindrical housing (12) adapted to be inserted in a receiving hole (11) formed in a top surface of a cylinder head (2) and having an internal thread (13) on an inner periphery thereof, an adjusting screw (15) having an external thread (14) on an outer periphery thereof and in threaded engagement with the internal thread (13) on the housing (12), and a return spring (17) biasing the adjusting screw (15) in a direction to protrude upwardly from the housing (12), said adjusting screw (15) having a protruding end (19) protruding from the housing (12) and adapted to support an arm (7) of a valve gear, characterized in that the surface of the protruding end (19) of the adjusting screw (15) is made of a material that does not react with oil additives of oil containing organic molybdenum.
 2. The lash adjuster of claim 1 wherein the adjusting screw (15) is made of a non-ferrous metal that does not react with oil additives of oil containing organic molybdenum.
 3. The lash adjuster of claim 1 wherein the adjusting screw (15) is made of iron, and wherein the protruding end (19) of the adjusting screw (15) is coated with a nonreactive film (22) selected from ceramic film, carbon film, oxide film, diamond-like carbon film, titanium nitride film and chromium nitride film.
 4. The lash adjuster of claim 1 wherein the adjusting screw (15) is made of iron, and wherein a nitride compound layer is formed on the protruding end (19) of the adjusting screw (15).
 5. The lash adjuster of claim 1 wherein the adjusting screw (15) is made of iron, and wherein nickel-phosphorus plating is provided on the protruding end (19) of the adjusting screw (15).
 6. The lash adjuster of claim 5 wherein the nickel-phosphorus plating is dispersion plating comprising a plating layer in which hard particulates are dispersed.
 7. The lash adjuster of claim 5 wherein the nickel-phosphorus plating is dispersion plating comprising a plating layer in which PTFE is dispersed.
 8. A lash adjuster comprising a cylindrical housing (12) adapted to be inserted in a receiving hole (11) formed in a top surface of a cylinder head (2) and having an internal thread (13) on an inner periphery thereof, an adjusting screw (15) having an external thread (14) on an outer periphery thereof and in threaded engagement with the internal thread (13) on the housing (12), a return spring (17) biasing the adjusting screw (15) in a direction to protrude upwardly from the housing (12), and an arm (7) having a recess (20) in which a protruding end (19) of the adjusting screw (15) protruding from the housing (12) is received, whereby the arm (7) is supported by the protruding end (19) of the adjusting screw (15), characterized in that the arm (7) is made of iron and the inner surface of the recess (20) of the arm (7) is made of a material that does not react with oil additives of oil containing organic molybdenum.
 9. The lash adjuster of claim 8 wherein the recess (20) of the arm (7) is coated with a nonreactive film (22) selected from ceramic film, carbon film, oxide film, diamond-like carbon film, titanium nitride film and chromium nitride film.
 10. The lash adjuster of claim 8 wherein a nitride compound layer is formed on the inner surface of the recess (20) of the arm (7).
 11. The lash adjuster of claim 8 wherein nickel-phosphorus plating is provided on the recess (20) of the arm (7).
 12. The lash adjuster of claim 11 wherein the nickel-phosphorus plating is dispersion plating comprising a plating layer in which hard particulates are dispersed.
 13. The lash adjuster of claim 11 wherein the nickel-phosphorus plating is dispersion plating comprising a plating layer in which PTFE is dispersed.
 14. A lash adjuster comprising a cylindrical housing (12) adapted to be inserted in a receiving hole (11) formed in a top surface of a cylinder head (2) and having an internal thread (13) on an inner periphery thereof, an adjusting screw (15) having an external thread (14) on an outer periphery thereof and in threaded engagement with the internal thread (13) on the housing (12), a return spring (17) biasing the adjusting screw (15) in a direction to protrude upwardly from the housing (12), and an arm (7) having a recess (62) in which a protruding end (61) of the adjusting screw (15) protruding from the housing (12) is received, wherein the protruding end (61) of the adjusting screw (15) is hemispherical in shape and is in contact with the recess (62) of the arm (7) at its radially outer portion, characterized in that as taken along a plane containing an axis (L) of the adjusting screw (15), the recess (62) of the arm (7) has a section whose radius of curvature (A2) is larger than the radius of curvature (A1) of the protruding end (61) of the adjusting screw (15), whereby the protruding end (61) possesses the wedging effect.
 15. The lash adjuster of claim 14 wherein the recess (62) of the arm (7) has an arcuate section as taken along a plane containing the axis (L) of the adjusting screw (15), and has centers of arc that are offset from the axis (L) of the adjusting screw (15) such that a radius of arc (A2) of the recess (62) intersects the axis (L) of the adjusting screw (15), and wherein the protruding end (61) of the adjusting screw (15) has an arcuate section having a center located on the axis (L) of the adjusting screw (15), as taken along a plane containing the axis (L) of the adjusting screw (15).
 16. The lash adjuster of claim 14 wherein a cutout (63) is formed in the inner surface of the recess (62) of the arm (7) at its central portion.
 17. The lash adjuster of claim 14 wherein the recess (72) of the arm (7) has an arcuate section having a center located on the axis (L) of the adjusting screw (15), as taken along a plane containing the axis (L) of the adjusting screw (15), and wherein the protruding end (71) of the adjusting screw (15) has an arcuate section as taken along a plane containing the axis (L) of the adjusting screw (15), and has centers of arc that are offset from the axis (L) of the adjusting screw such that a radius of arc (B1) of the protruding end (71) is separate from the axis (L) of the adjusting screw (15).
 18. The lash adjuster of claim 17 wherein a cutout (63) is formed in the inner surface of the recess (72) of the arm (7) at its central portion.
 19. The lash adjuster of claim 14 wherein the recess (82) of the arm (7) has an arcuate section as taken along a plane containing the axis (L) of the adjusting screw (15), and has centers of arc that are offset from the axis (L) of the adjusting screw (15) such that a radius of arc (C2) of the recess (82) intersects the axis (L) of the adjusting screw (15), and wherein the protruding end (81) of the adjusting screw (15) has an arcuate section as taken along a plane containing the axis (L) of the adjusting screw (15), and has centers of arc that are offset from the axis (L) of the adjusting screw such that a radius of arc (C1) of the protruding end (81) is separate from the axis (L) of the adjusting screw (15).
 20. The lash adjuster of claim 19 wherein a cutout (63) is formed in the inner surface of the recess (82) of the arm (7) at its central portion.
 21. A lash adjuster comprising a cylindrical housing (12) adapted to be inserted in a receiving hole (11) formed in a top surface of a cylinder head (2) and having an internal thread (13) on an inner periphery thereof, an adjusting screw (15) having an external thread (14) on an outer periphery thereof and in threaded engagement with the internal thread (13) on the housing (12), and a return spring (17) biasing the adjusting screw (15) in a direction to protrude upwardly from the housing (12), said adjusting screw (15) having a hemispherical protruding end (91) protruding from the housing (12) and adapted to pivotally support an arm (7) of a valve gear, characterized in that a recessed surface (93) or a flat surface is formed on the protruding end (91) of the adjusting screw (15) at its central portion, the recessed surface (93) or the flat surface having a diameter (D1) which is half or more than half the diameter (D2) of the protruding end (91) of the adjusting screw (15). 22-24. (canceled)
 25. A lash adjuster of claim 21 wherein the adjusting screw comprises a pivot member (15A) axially slidably inserted in the housing (12), an externally threaded member (15B) supporting an end of the pivot member (15A) inserted in the housing (12) and having said external thread (14) on its outer periphery, and an elastic member (15C) disposed between the pivot member (15A) and the externally threaded member (15B).
 26. A lash adjuster of claim 1 wherein the adjusting screw comprises a pivot member (15A) axially slidably inserted in the housing (12), an externally threaded member (15B) supporting an end of the pivot member (15A) inserted in the housing (12) and having said external thread (14) on its outer periphery, and an elastic member (15C) disposed between the pivot member (15A) and the externally threaded member (15B).
 27. A lash adjuster of claim 8 wherein the adjusting screw comprises a pivot member (15A) axially slidably inserted in the housing (12), an externally threaded member (15B) supporting an end of the pivot member (15A) inserted in the housing (12) and having said external thread (14) on its outer periphery, and an elastic member (15C) disposed between the pivot member (15A) and the externally threaded member (15B).
 28. A lash adjuster of claim 14 wherein the adjusting screw comprises a pivot member (15A) axially slidably inserted in the housing (12), an externally threaded member (15B) supporting an end of the pivot member (15A) inserted in the housing (12) and having said external thread (14) on its outer periphery, and an elastic member (15C) disposed between the pivot member (15A) and the externally threaded member (15B). 